This invention relates generally to the field of rope or cable tensioning devices and more specifically to a winch assembly for use with synthetic or organic ropes.
Winch drums are widely used for tensioning and storing wire ropes. They are simple, robust and, if properly designed, long-lasting. They are not, however, compatible with synthetic ropes which can be crushed by the compressive forces that accumulate as rope is spooled onto the winch drum under tension.
Steel used for wire ropes is nearly isotropic and resists crushing well. It has a high tensile strength and stiffness along with good wear resistance. Modern synthetic ropes, made from materials such as Kevlar™ and Vectran™, have even higher strength and stiffness in the longitudinal direction with much lower weight and vastly superior corrosion resistance. (Kevlar is a trade mark of DuPont de Nemours Co. Vectran is a trade mark of Hoechst Celanese Corp.) For applications where weight, strength, stiffness, corrosion resistance or rope flexibility are important synthetic ropes are preferred. Examples are elevators, hoists, cranes, tensioners in deep-sea rigs and lines used in Underway Replenishment at sea. Unfortunately these synthetics are highly anisotropic and can be easily damaged by stresses oriented orthogonal to the fiber direction. They are also subject to rapid wear and fibrilation due to stresses that arise when fibers move within the rope. For these reasons it is important to carefully manage stress within a synthetic rope in any application.
Elevators have successfully used synthetic ropes for considerable time. This application is, however, distinct from winches since there is no spooling of rope—crushing loads are therefore of no concern. Rope wear and the application of large traction forces to the rope are, however, concerns that are shared with winch systems.
De Angelis et al (U.S. Pat. No. 5,566,786-1996) is one of many patents that describes a synthetic rope for use with elevators or lifts. Typical of these patents, De Angelis makes claims for structures that are “for the protection of the fibers . . . ”. Subsequent patents by De Angelis (U.S. Pat. Nos. 6,318,504 and 6,397,574) teach the use of “an elastic intersheath between the layers of strands . . . to assist in transmitting torque within the rope over a large area.” The need for shear transfer from the outer surface to the interior of the rope has been recognized but the inventor fails to appreciate that this problem can be better accomplished by using a large contact area between the traction sheave and the rope. Increasing the sheave diameter has no effect on the tension that can be applied but reduces the shear stress within the rope.
It is well known that the tension that can be applied to a rope by a traction sheave is expressed by the ratio T1/T2=eμα, where T1 and T2 are the tensions of the rope entering and exiting the sheave, μ is the coefficient of friction and α is the wrap angle of the rope around the sheave in radians. Both friction coefficient and wrap angle have been exploited as a means to increase the load that can be transmitted by a traction sheave.
In O'Donnell et al (U.S. Pat. No. 6,164,053) “the material for the jacket and sheave liner are selected to optimize the coefficient of friction between the hoist rope and traction sheave.” Other inventors have claimed high-friction coatings or surface roughening to achieve the same result. In Heikkinen (U.S. Pat. No. 5,076,398) and in other patents the wrap angle is increased to nearly 270° by using one or more idler sheaves that are displaced from a traction sheave by a short distance along their rotational axes. This approach can cause excessive rope wear unless the sheave's groove is modified, which is undesirable as it allows excessive distortion of the rope with consequent internal wear. Ungrooved drums have been proposed by, for example, Salmon (U.S. Pat. No. 5,186,283) to achieve wrap angles in excess of 360° but eliminating the groove greatly increases rope distortion and wear.
In a patent by Köster (U.S. Pat. No. 6,193,017) multiple traction sheaves are used to tension a rope supporting an elevator. The sheaves are arranged such that wrap angles of more than 180° can be achieved. With this design very high tension can be achieved and the counterweight can be eliminated. (Hollowell et al contains similar teaching in U.S. Pat. No. 6,193,016 but these claims were anticipated by Köster).
All of the cited patents seek to increase the amount of tension that can be applied to a rope however none address the crushing that occurs on winch drums. In addition little thought has been given to how the sheaves can be driven when more than one are used to apply traction. In one embodiment Köster teaches that “The traction sheaves . . . may be driven via a common motor, with the use of a suitable transmission gearing . . . ” although it must be noted that this is not the preferred embodiment. Such an arrangement ignores the change in length that occurs with changes in rope tension—with the result that slip is introduced between the rope and the sheaves if these latter are not independently driven. Hollowell et al make a similar statement: the multiple traction sheaves are driven by “one or more prime movers . . . ”
No identified prior art addresses the problem that exists in wrapping a rope, particularly a synthetic rope, onto a drum at high tension. Motors and controllers that would allow tension or rotational velocity to be independently controlled over multiple traction sheaves and a winch drum have not been disclosed. The prior art is insufficient to design a winch system that is compatible with synthetic ropes.